The AFM is a well-known device in which the topography of a sample is modified or sensed by a probe or probe mounted on the end of a microfabricated cantilever. As the sample is scanned, the interaction of atomic forces between the probe and the sample surface causes pivotal deflection of the cantilever. The topography of the sample may be determined by detecting this deflection of the probe. By controlling the deflection of the cantilever and the physical properties of the probe the surface topography may be modified to produce a pattern on the sample.
Lithography is the process for producing a pattern of two dimensional shapes made by drawing primitives such as lines and pixels within a layer of material, such as, for example, a resist coated on a semiconductor device. Conventional photolithography is running into severe problems as the feature size is reduced below 65 nm. These problems arise from fundamental issues such as sources for the low wavelength of light, photoacid migration, photoresist collapse, lens system quality for low wavelength light and masks cost. To overcome these issues an alternative approach to the generation of sub-50 nm features is required.
One approach to addressing these issues is to use a scanning probe-based lithography (SPL) device. In this strategy a probe is raster scanned across a resist surface and brought to locally interact with the resist material. By this interaction resist material is removed or changed in a way that it can be developed.
SPL is described in detail in Chemical Reviews, 1997, Volume 97 pages 1195 to 1230, “Nanometer-scale Surface Modification Using the Scanning Probe microscope: Progress since 1991”, Nyffenegger et al. and the references cited therein. In particular, the article describes use of organic materials as lithographically active layers (see pages 1219 and 1220) which have been used in mechanical modicon experiments to generate line patterns in the organic resist material.
In the mechanical modification experiments the drawing primitives are physically realized as indentations in the polymer layer created by heating the cantilever probe and with the application of force pressing this probe into the polymer. The probe is heated by passing a current through a resistive heater integrated in the cantilever, directly behind the probe. Some of the heat generated in the resistor is conducted through the probe and into the polymer layer, locally heating a small volume of the polymer. If sufficient heat is transferred to raise the temperature of the polymer above a certain temperature (which is dependent on the chosen polymer), the polymer softens and the probe sinks in, creating an indentation or line. Examples of organic resist materials used were poly(methyl methacrylate) (PMMA) and poly(glycidyl methacrylate) PGMA.
There are a number of problems with the known implementations of SPL. These problems include difficulty with crossing lines if material is not properly removed but only squashed to the side by the interacting probe, contamination of the probe if material is not removed, and if a layer is structured by indenting or scratching with the probe, the probe tends to wear.
The efficiency of forming indentations is therefore critically dependent on the nature of the polymeric thin film used as the resist. Desirable attributes of the polymeric thin film are ‘softness’ and deformability during the writing phase, stability, toughness and resistance to wear during mechanical and chemical processing. A hard polymer with a high melting point will be difficult to soften sufficiently for the probe to sink in and form the pit during the writing process. Linear polymers such as PMMA have been found to have suitable writing temperatures and the force required on the probe to form the indentation is acceptably low for the required probe performance and power consumption; however, rim formation around the indentations and the erasing of indentations when writing indentations in close proximity is a problem. Such problems would be overcome by using a harder crosslinked polymer but this would lead to unacceptable probe wear rate during writing.
An example of scanning thermal lithography using polymers which undergo selective crosslinking is disclosed at http://www.eecs.umich.edu/˜basualpages/SThL. html.
The present invention seeks to overcome these problems by using a class of polymers which under controlled conditions have the characteristics of linear polymers and are thus suitable for the writing phase but have the characteristics of crosslinked polymers during subsequent processing steps.